Darwin noticed it and yet it's been ignored for the past 130 years. Male animals are often decorative or showy, from the brilliant plumage of birds of paradise to the steely antlers of male stag beetles. Females, in contrast, are drab creatures who lurk in the undergrowth.
At least, that's been the received wisdom. But Dr Trond Amundsen, from the Norwegian University of Science and Technology in Trondheim, is attempting to redress that balance. According to him, many females are brightly coloured and almost nobody has asked why; researchers have concentrated, instead, on solving the problem of ornamented males. "It's so striking that scientists have been faced with tremendous variability and beauty in both sexes - and they've only looked at one," says Dr Amundsen.
The current explanation for huge horns of stags or trailing feathers of peacocks is that they are to attract females. Moreover, they can be indicators of good genes: only healthy males, relatively free from parasites, sport large symmetrical appendages or have glossy fur and feathers.
But extravagant ornamentation is not uncommon in females: some species of sea bird such as auks or cormorants have colourful beaks or conspicuous crests; many female fish are colourful, as are dragonflies and beetles; female reindeer and antelope possess horns as well as the males.
However, in general, if females are brightly coloured they tend to be similar to males. Magpies, for instance, are a classic example, since both sexes have striking black and white plumage and long tails. Only in a few species, such as the two-spotted goby, the peacock wrasse, and sunangel hummingbirds, do females branch out on their own.
The great Victorian naturalist Alfred Wallace, in an essay written shortly before Darwin's Descent of Man was published, suggested that females were drab because they needed to be camouflaged when incubating eggs. In species where the female is hidden from predators, such as parrots that nest within hollow trees, females can be as colourful as their mates.
Darwin wasn't convinced, and thought that perhaps female coloration was linked to male decoration. Female offspring inherit their father's genes for bright colours or big fins. "We shouldn't assume that female ornamentation is only a by-product of the males' ornamentation," says Dr Amundsen. "There must be some correlation," he adds. "Males and females share the same genes, and it's striking that where females are ornamented, they are like the males. But this could be a starting point - sharing the male's genes is just the building blocks for female ornamentation. …